The invention relates to a method for performing pyrolysis, wherein solid fuel in the form of particles is supplied to a pyrolyzer, and pyrolysis conditions are maintained in the pyrolyzer in order to separate condensable gaseous substances from the fuel. The invention further relates to a pyrolysis apparatus.
Pyrolysis refers to the conversion of fuel under inert conditions and at a high temperature to gaseous form, which during condensation forms oily liquid that comprises different organic compounds. In connection with pyrolysis, inert conditions refer to oxygen-free conditions, wherein combustion of fuel is avoided. Tar distillation is one example of a pyrolysis process known for ages.
In the pyrolysis process, the fuel is pyrolyzed, the gaseous compounds formed in the reaction are separated from carbonization residue, and they are condensed into pyrolysis oil which may be used, for example, as fuel, or it may be processed further into different chemicals. Production of pyrolysis oil from different bio-based, for example wood-based fuels has been studied with the purpose of replacing coal and heavy fuel oil with it. One advantage of pyrolysis oil is its easy transportation in comparison to biomass that is difficult to transport, when the energy content of fuels are taken into account.
Examples on the development of pyrolysis processes include many patent publications, such as U.S. Pat. No. 4,891,459, U.S. Pat. No. 5,728,271, EP 513051, U.S. Pat. No. 6,814,940, WO 97/06886, WO 02/083816, and WO 03/106590.
A particular set is formed by publications, in which a pyrolyzer is placed in connection with a fluidized bed boiler that burns fuel, as presented e.g. in patent FI 117512 as well as patents FI 122858 and FI 122778 and the corresponding US application publications US 20090242376 ja US 20090242377. In these, the energy content of hot inert bed material (sand) taken from the fluidized bed boiler is utilized for performing endothermic pyrolysis. The bed material which has released the required heat in the pyrolysis in the pyrolyzer is returned to the furnace of the fluidized bed boiler. At the same time, residual carbon (coke) from the pyrolysis process, i.e. pyrolyzed fuel residue, is carried with the bed material to the furnace where it burns, whereby it partly replaces the fuel of the boiler. Non-condensable gases formed in the pyrolysis process can also be conveyed to the furnace for combustion. The boiler can be a circulating fluidized bed boiler (CFB) or a bubbling fluidized bed boiler (BFB).
Pyrolysis integrated in a fluidized bed boiler provides advantages in the process technology, but a disadvantage is that it may be difficult to find a suitable location for the pyrolyzer in the cramped boiler environment. That is to say, the pyrolyzer has to be close to the boiler so that the bed material does not cool too much during the passage between the boiler and the pyrolyzer. Furthermore, the pyrolysis process is dependent on the load of the boiler. Moreover, the use of a boiler operating on the principle of a bubbling fluidized bed (BFB) in combination with a pyrolyzer is hampered because residual carbon from the pyrolysis process may fly with fluidizing air out of the bed, whereby the energy contained in it is wasted.